Printing presses, and particularly web fed lithographic offset printing presses, typically employ a steel fountain roll which turns in a reservoir of ink. A flexible or segmented doctor blade presses against this roll, with adjustable, controllable pressure, for example in different zones, to control the thickness of the ink film acquired by the roll as it dips into an ink trough or reservoir. Blade adjustment screws, or similar devices, permit control of the thickness of the ink film picked up by the fountain roll to be varied across the width of the press in order to fit selected print requirements across the width of the substrate on which printing is to be carried out.
The ink train itself includes a plurality of rollers or cylinders, some of which may oscillate axially. Ink is transferred from the fountain roll to the ink train in accordance with the system to which the present invention relates by a free-turning ductor roller, which oscillates back and forth between the fountain roll and the first ink transfer roll of the ink train. The ductor is moved by a mechanical drive, energized by the gear train of the printing press, to engage, alternately, with the fountain roll and the first ink transfer roll or roller of the ink train.
The amount of ink delivered to the ink train by the ductor roller depends on the degree of angular advancement of the fountain roll during the time that the ductor roller is in contact with the fountain roll, as well as on the thickness of the ink across the axial length of the fountain roll, and hence of the ductor roller. In some presses, the fountain roll turns continuously; a variable speed drive is provided, the variable speed drive changing speed as a function of and in dependence on the operating speed of the printing press. In addition, the speed ratio between the fountain roll and the press speed can be changed, to thereby vary the amount of ink being transferred, by changing the angle of rotation of the fountain roll during which the ductor roller is in contact therewith.
Ink is distributed from the first ink transfer roller through an array of ink distribution rollers to the plate cylinder of the printing machine. The ink, after inking the plate, is then offset on a blanket cylinder for printing on a substrate web, typically paper. If the paper has the characteristic that it can shed fibers, fluff or lint, such fibers, fluff or lint can be transferred back to the ink supply through the various ink distribution rollers and, eventually, can reach the ink reservoir or ink trough via the intermittently operatating or oscillating ductor roller. Fluff, lint, dust or other contaminants can then collect within the ink trough which, over time, can cause changes in the amount of ink being transferred.
To overcome the problem of change in ink transfer due to accumulation of contaminants in the ink trough, it has been proposed to rotate the fountain roll only during the time that the ductor roller is in contact with it. This intermittent motion, it has been found, prevents build-up of contaminants in selected regions of the ink trough, and hence on the fountain roller, which may cause striping or lines on the eventual image being printed. Intermittent motion of the fountain roller, in the past, has been obtained by various mechanisms, for example by an adjustable-length crank mechanism driven by the press, a gear train which includes gearing with gaps along the circumference for intermittent operation and which, even, permits reverse rotation of the fountain roller (see German Patent 37 16 679, Nawrath) or by a Maltese cross drive (see German Published Application 5 1 188 616, Norlin).
The inkers as known utilize the time during which the ductor roller is out-of-contact with the fountain roller as a dead or stop time for the fountain roller; thus, the fountain roller is driven only during the interval of engagement or contact with the ductor roller. This intermittent movement has been found to prevent accumulation of contaminants between the ink film control elements, typically doctor blades or doctor blade elements, and the fountain roller.
The apparatus to ensure intermittent operation of the rollers is complex and expensive, and causes maintenance problems.